The present invention relates to a fuel cell including an enzyme immobilized on at least one of a positive electrode and a negative electrode and to an electronic apparatus using the fuel cell.
Fuel cells have a structure in which a positive electrode (oxidizer electrode) and a negative electrode (fuel electrode) are opposed to each other with an electrolyte (proton conductor) provided therebetween. In a conventional fuel cell, fuel (hydrogen) supplied to a negative electrode is decomposed into electrons and protons (H+) by oxidation, the electrons are supplied to the negative electrode, and H+ moves to the positive electrode through the electrolyte. On the positive electrode, H+ reacts with oxygen supplied from the outside and the electrons supplied from the negative electrode through an external circuit to produce H2O.
Therefore, a fuel cell is a high-efficiency generating apparatus which directly converts chemical energy possessed by fuel to electric energy, and is capable of utilizing, with high efficiency, electric energy from chemical energy possessed by fossil fuel, such as natural gas, petroleum, and coal, regardless of an operation place and operation time. Consequently, fuel cells have been actively researched and developed as applications to large-scale power generation. For example, an actual performance has proved that a fuel cell provided on a space shuttle can supply electric power and water for crews and is a clean generating apparatus.
Further, fuel cells such as solid polymer-type fuel cells, which show a relatively low operation temperature range from room temperature to about 90° C., have recently been developed and attracted attention. Therefore, not only application to large-scale power generation but also application to small systems such as driving power supplies of automobiles and portable power supplies of personal computers and mobile devices are being searched for.
Thus, fuel cells are considered to be widely used for applications including large-scale power generation and small-scale power generation and attract much attention as high-efficiency generating apparatuses. However, fuel cells generally use, as fuel, natural gas, petroleum, or coal which is converted to hydrogen gas by a reformer, and thus have various problems of the consumption of limited resources, the need to heat to a high temperature, the need for an expensive noble metal catalyst such as platinum (Pt), and the like. In addition, even when hydrogen gas or methanol is directly used as fuel, it is necessary to take caution to handling thereof.
Therefore, attention is paid to the fact that biological metabolism in living organisms is a high-efficiency energy conversion mechanism, and its application to fuel cells has been proposed. The biological metabolism includes aspiration and photosynthesis taking place in microorganism cells. The biological metabolism has the characteristic that the generation efficiency is very high, and reaction proceeds under mild conditions such as room temperature.
For example, aspiration is a mechanism in which nutrients such as saccharides, fat, and proteins are taken into microorganisms or cells, and the chemical energy thereof is converted to oxidation-reduction energy, i.e., electric energy, by a process of producing carbon dioxide (CO2) through a glycolytic system including various enzyme reaction steps and a tricarboxylic acid (TCA) cycle, in which nicotinamide-adenine dinucleotide (NAD+) is reduced to reduced nicotinamide-adenine dinucleotide (NADH). Further, in an electron transfer system, the electric energy of NADH is converted directly into proton gradient electric energy, and oxygen is reduced, producing water. The electric energy obtained in this mechanism is utilized for reaction necessary for producing ATP from adenosine diphosphate (ADP) through an adenosine triphosphate (ATP) synthetase, and ATP is used for a reaction necessary for growing microorganisms or cells. Such energy conversion takes place in plasmasol and mitochondoria.
In addition, photosynthesis is a mechanism in which light energy is taken in, and water is oxidized to produce oxygen by a process of converting to electric energy by reducing nicotinamide-adenine dinucleotide phosphate (NADP+) to reduced nicotinamide-adenine dinucleotide phosphate (NADPH) through an electron transfer system. The electric energy is utilized for a carbon immobilization reaction in which CO2 is taken in to synthesize carbohydrates.
As a technique for utilizing the above-mentioned biological metabolism in a fuel cell, there has been reported a microbial cell in which electric energy generated in microorganisms is taken out from microorganisms through an electron mediator, and the electrons are supplied to an electrode to produce a current (refer to, for example, Japanese Unexamined Patent Application Publication No. 2000-133297).
However, there are many unnecessary reactions other than the desired reaction for converting chemical energy to electric energy in microorganisms and cells, and thus electric energy is consumed for an undesired reaction in the above-described method, thereby failing to exhibit a sufficient energy conversion efficiency.
Therefore, there have been proposed fuel cells (biofuel cells) in which only a desired reaction is effected using an enzyme (refer to, for example, Japanese Unexamined Patent Application Publication No. 2003-282124, Japanese Unexamined Patent Application Publication No. 2004-71559, Japanese Unexamined Patent Application Publication No. 2005-13210, Japanese Unexamined Patent Application Publication No. 2005-310613, Japanese Unexamined Patent Application Publication No. 2006-24555, Japanese Unexamined Patent Application Publication No. 2006-49215, Japanese Unexamined Patent Application Publication No. 2006-93090, Japanese Unexamined Patent Application Publication No. 2006-127957, and Japanese Unexamined Patent Application Publication No. 2006-156354). As the biofuel cells, there have been developed biofuel cells in which fuel is decomposed into protons and electrons by an enzyme, an alcohol such as methanol or ethanol, or a monosaccharide such as glucose being used as the fuel.
The biofuel cell includes an electrolyte generally containing a buffer material (buffer solution). This is intended for controlling pH to near pH at which an enzyme easily functions with the buffer material because an enzyme used as a catalyst is very sensitive to pH of a solution. As the buffer material, sodium dihydrogen phosphate (NaH2PO4), 3-(N-morpholino)propanesulfonic acid (MOPS), N-2-hydroxyethylpiperazine-N′-2-ethanesulfonic acid (HEPES), and the like are conventionally used. The concentration of the buffer material is generally 0.1 M or less. This is because generally, the concentration of the buffer material is diluted to the minimum level required for maintaining pH constant, and appropriate inorganic ions or organic ions are added for creating approximately physiological conditions.
However, according to research conducted by the inventors, when in the above-described conventional biofuel cell using NaH2PO4, MOPS, HEPES, or the like as the buffer material contained in the electrolyte, an enzyme immobilized on a high-surface-area electrode such as porous carbon is used or the concentration of immobilized enzyme is increased to increase output, the pH of the electrolyte around the enzyme deviates from optimum pH due to the insufficient buffer ability, and the ability inherent in the enzyme cannot be sufficiently exhibited.
Accordingly, it is desirable to provide a fuel cell having excellent performance and capable of producing a sufficient buffer ability even in a high-output operation and sufficiently exhibiting the ability inherent in an enzyme when an enzyme is immobilized on at least one of a positive electrode and a negative electrode.
It is further desirable to provide an electronic apparatus using the above-described excellent fuel cell.